The present invention generally relates to erasing devices for erasing charge remaining on a photosensitive member of a copying apparatus, a laser printer or the like.
Conventionally, in image forming apparatuses of a type in which corona charging, image exposure and development are performed on a photosensitive member so as to form a toner image on the photosensitive member such that the toner image is transferred onto a paper sheet, for example, a copying apparatus, a printer, etc., various erasing devices are employed so as to erase charge remaining on the photosensitive member after transfer of the toner image. For example, a tungsten lamp is used as a light source or a monochromatic light emitting diode is used as a light source.
Meanwhile, since a number of defects are present in a photosensitive layer of the photosensitive member, carrier is trapped in the defects and thus, it is impossible to completely erase residual charge. Hence, even after charge erasing, the photosensitive member has slight potential. Namely, when light is irradiated over the photosensitive member to which charge of negative polarity is imparted by a corona charger, holes and electrons are generated in the photosensitive layer at an irradiated portion of the photosensitive member and the holes are displaced towards a surface of the photosensitive member so as to cancel negative charge on the surface of the photosensitive member. On the other hand, the electrons are displaced towards a grounded substrate. At this time, a portion of the moving holes are trapped in the defects during travel of the holes and thus, surface potential of the photosensitive member does not drop completely, thereby resulting in residual potential.
Therefore, such problems arise that residual potential referred to above rises gradually through repetition of corona charging and exposure and charge erasing effect varies according to especially, environmental conditions such as temperature and humidity, thus adversely affecting an image. Variations of charge erasing effect due to change of environmental conditions is conspicuous in the case of an organic photosensitive member.